Heretofore, radio frequency (RF) microelectromechanical system (MEMS) switches have utilized an electrostatic force or electrothermal actuation to actuate the RF MEMS switch. In a typical electrostatic RF MEMS switch, at least 30 volts may be required to open and close the switch. Consequently, the switch is not suitable for applications such as commercial handheld products, which typically operate on 3 volts or less. The electrostatic RF MEMS switch also is limited in its operation, as it can only be open or closed, that is, either in contact or not in contact. For this reason, the electrostatic RF MEMS switch is not suitable as a tunable capacitor, as such devices typically require controlled variance in the displacement of the actuation beam.
Electrostatic RF MEMS switches also suffer from a well known problem known as stiction, which occurs when surface tension forces are higher than the spring restoring force of the actuator beam. Stiction may be caused by a wet etching process used during fabrication, which may leave some moisture or meniscus which pulls the beam towards the electrode and prevents the beam from releasing. Alternatively, or additionally, stiction may occur during operation, whereby the beam stays in a deflected position due to capillary forces, electrostatic attraction, or direct chemical bonding. Stiction is a major problem of electrostatic RF MEMS switches, oftentimes rendering the switch inoperable.
Electrostatic RF MEMS devices also may require additional fabrication steps, particularly RF MEMS devices requiring high quality frequency performance. Such devices are typically fabricated using RF-compatible substrate materials such as GaAs, ceramics, and high resistivity silicon. According to one technique, an RF circuit is fabricated from an RF-compatible substrate and an actuator is fabricated on a silicon wafer, and then the circuit and actuator are assembled using flip chip technology. Since the silicon has a low resistivity which may interfere with the RF performance of the circuit, typically a switch manufacturer removes the silicon, leaving only the actuator on the RF circuit. For high volume applications, this additional silicon removal step may be quite costly.
Electrothermal actuated devices also are not without drawbacks. The function of an electrothermal actuator depends on the mismatching of the thermal expansion rates of different dimensioned actuator beams. The electrothermal actuator has some limitations such as slower tuning and more space requirements. Moreover, the manufacturing process of electrothermal actuators involves critical design considerations such as temperature distribution and heat sink placement. In operation, the beam is heated by applying a current (Joule heating), causing the beam to move due to the differing expansion rates of the materials forming the beam. Once actuated, however, the beam must cool down in order to return to its original position. Controllably cooling down the beam is difficult, as the amount of time to sufficiently cool the beam oftentimes is not ascertainable or is met with inconsistent results. Although the actuator may be made smaller to reduce its cooling time, the cooling time still cannot be controlled effectively to vary the interelectrode spacing and hence the capacitance between the electrodes. For this reason, the electrothermal MEMS switch is usually employed as a one-way switch rather than a two-way switch or a tunable capacitor.